The invention is situated in the general context of fabrication of a Semiconductor on Insulator (SeOI) type substrate by means of the SmartCut™ method, and of recycling the remainder of the donor substrate.
The SmartCut™ method is described in detailed manner for example in U.S. Pat. No. 5,374,564. This method enables a layer to be transferred from a donor substrate to a receiver substrate.
With reference to FIG. 1, it should be recalled that it comprises the following steps:
1) Oxidation of the donor substrate 10 or receiver substrate 30 to form an oxide layer 20;
2) Ion implantation in the donor substrate 10 to form a weakened zone 12;
3) Bonding of the donor substrate 10 onto the receiver substrate 30, the oxide layer 20 being located at the interface;
4) Fracturing the donor substrate 10 in the weakened zone 12 and transfer of a layer 11 from the donor substrate 10 to the receiver substrate 30;
5) Finishing treatment of the SeOI 1 formed in this way.
It is advantageous to recycle the remainder 13 of the donor substrate to use the latter in a new implementation of the SmartCut™ method. After polishing, it can be re-used as donor substrate: this is then referred to as “top for top” recycling, or it can be re-used as receiver substrate, in which case “top for base” recycling is referred to.
Recycling in particular comprises polishing of at least one of the faces of the substrate to remove the material damaged by implantation and fracturing. The thickness removed may be a few microns.
It should be stipulated that the invention involves the latter case, i.e. recycling of the remainder of the donor substrate as receiver substrate. The donor substrate may not be “new”—it may in fact have been previously used as donor substrate and recycled as donor substrate, but a limited number of times so as not to be too damaged.
When the remainder of the donor substrate is recycled to constitute a new donor substrate, the succession of SmartCut™ cycles involving thermal treatments at medium (i.e. less than 1000° C.) or low (i.e. less than 600° C.) temperatures, in particular when performing oxidation and fracturing, does in fact generate defects in the substrate which may limit use thereof.
Recycling of the remainder of the donor substrate as receiver substrate on the other hand enables the substrate to be subjected to two SmartCut™ cycles only, the first as donor substrate and the second as recycled receiver substrate.
This recycling mode therefore enables the number of thermal treatments undergone by the substrate to be limited, thereby minimizing damage of the latter.
However, even in this case, on account of the thermal treatments it has undergone during implementation of the SmartCut™ method and depending on its initial properties, the recycled substrate may present denser and/or larger defects than those of the initial donor substrate. These defects are distributed in the volume of the material and do not necessarily propagate to the surface. The recycled substrate can therefore be of less good quality than the initial donor substrate.
These defects are referred to as Bulk Micro Defects (BMD). These defects correspond typically to oxygen precipitates within the substrate. Formation of these precipitates is induced by the thermal treatments undergone by the substrate and also depends on the initial properties of the material. More precisely, the density of BMD defects is considered to be linked to the temperature gradient applied, whereas the mean size of these defects is linked to the maximum treatment temperature.
However, depending on the intended use of the SeOI formed in this way, electronic component manufacturers define specifications imposing a maximum value for the BMD defect density and/or a maximum value for the size of these defects.
A first object of the invention is therefore to improve the quality of the recycled substrate. A second object of the invention is to enable a recycled substrate of required thickness to be obtained at the end of the polishing step which can remove a consequent material thickness. The invention further has the object of optimizing the quality of the SeOI on completion of the first cycle of SmartCut™.